1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile and the like.
2. Related Background Art
Some image forming apparatuses such as copying machines, printers and facsimiles have a multi-mode in which plural image forming operations are effected regarding a single sheet and a both-face mode in which image forming operations are effected regarding both surfaces of each sheet. In the multi-mode and the both-face mode, sheets on which images were formed are temporarily stacked on an intermediate tray in the image forming apparatus, and then, each sheet is re-supplied from the intermediate tray to an image forming portion. In this case, the sheet first image formation of which was finished in the multi-mode is placed on the intermediate tray with an imaged surface facing downwardly; whereas, the sheet first image formation of which was finished in the both-face mode is rested on the intermediate tray with an imaged surface facing upwardly.
FIG. 11 shows a conventional image forming apparatus (copying machine) 100B having a multi-mode and a both-face mode.
The image forming apparatus 100B is provided with a plurality of sheet supply cassettes 1 to 4 of front loading type. Sheets S1 to S4 having different sizes are contained in the sheet supply cassettes 1 to 4. For example, the sheets S1 having A5 size or statement size are stacked and contained in the cassette 1, the sheets S2 having A4 size, B5 size or letter size are stacked and contained in the cassette 2, the sheets S3 having A4R size, B5R size or letter R size are stacked and contained in the cassette 3, and the sheets S4 having B4 size or legal size are stacked and contained in the cassette 4.
In the image forming apparatus 100B, the sheets S1 to S4 in the sheet supply cassettes 1 to 4 are selectively supplied. The sheets S1 to S4 stacked and contained in the sheet supply cassettes 1 to 4 are supplied one by one from an uppermost sheet by means of respective sheet supply rollers 5 rotated in an ant-clockwise direction in FIG. 11. The sheets S fed out by the corresponding sheet supply roller 5 are separated by a pair of separation rollers 6 so that only the uppermost sheet is sent to a pair of register rollers 13 (which are now stopped) through convey rollers 7 to 12.
The convey rollers 7 to 11 convey the sheet S until a predetermined loop is formed in the sheet after a tip end of the sheet abuts against a nip between the pair of register rollers 13. By forming such a loop, skew-feed of the sheet S is corrected.
An electrostatic latent image is formed on a photosensitive drum (electrophotographic photosensitive member) 14 rotated in a clockwise direction in FIG. 11, by an exposure means utilizing a laser beam, and the latent image is developed with developer (toner) from a developing means to form a toner image. The sheet S (skew-feed of which was corrected) is sent to a transfer station between the photosensitive drum (image bearing member) 14 and a transfer charger (transfer means) 15, by the pair of register rollers 13 which start to be rotated at a timing for aligning the tip end of the sheet with the toner image. While the sheet is passing through the transfer station, the toner image on the photosensitive drum 14 is transferred onto the sheet by the transfer charger 15.
The sheet S to which the toner image was transferred is sent to a pair of fixing rollers 17 by a convey belt 16. While the sheet is passing through between the pair of fixing rollers 17, the toner image is fixed to the sheet by heat and pressure.
Thereafter, the sheet S is conveyed by a pair of convey rollers 18. In a normal mode, the sheet is sent to a pair of discharge rollers 20 through a sheet discharge path 19 and then is discharged onto a sheet discharge tray 21 out of the apparatus by the pair of discharge rollers 20. In the normal mode, as shown by the solid line, a flapper 22 opens the sheet discharge path 19 and opens a vertical path 23.
In a multi-mode or both-face mode, the sheet S conveyed by the pair of convey rollers 18 is sent to the vertical path 23 communicating with an intermediate tray 29. In this case, the flapper 22 is shifted to a position as shown by the broken line to close the sheet discharge path 19 and open the vertical path 23.
Explaining with reference to FIGS. 11, 12A, 12B and 12C, in the multi-mode, the sheet S sent into the vertical path 23 is sent to a convey-in path 27 by pairs of convey rollers 24 to 26 and then is sent to a discharge path 60 above the intermediate tray 29 by a pair of convey-in rollers 28. Then, the sheet S is discharged onto the intermediate tray 29 from one of pairs of discharge rollers 61 to 64 disposed in the discharge path 60 along a sheet conveying direction, with an imaged surface of the sheet facing downwardly.
When a sheet S having a maximum size in the sheet conveying direction is used, in accordance with the sheet size, a first flapper 66 is rotated upwardly, thereby discharging the sheet S from a discharge opening 41. When a sheet S having second large size is used, a second flapper 67 is rotated upwardly thereby discharging the sheet S from the first pair of discharge rollers 61. When a sheet S having third large size is used, a third flapper 68 is rotated upwardly to thereby discharge the sheet S from the second pair of discharge rollers 62. When a sheet S having fourth large size is used, a fourth flapper 69 is rotated upwardly, thereby discharging the sheet S from the third pair of discharge rollers 63. When a sheet S having smallest size is used, a fifth flapper 70 is rotated upwardly, thereby discharging the sheet S from the fourth pair of discharge rollers 64 (refer to FIG. 12A).
As shown in FIGS. 12B and 12C, the sheet discharged on the intermediate tray 29 is conveyed by an auxiliary roller 71 rotated in an anti-clockwise direction until the tip end of the sheet abuts against a rotatable shutter 44 which is now cocked. In this case the tip end of the sheet is guided by a movable sheet guide 72 to suppress a curl. The auxiliary roller 71 is positioned at an upper waiting position when the sheet is discharged to permit the discharging of the sheet and, immediately after the sheet was discharged, the roller 71 is lowered to convey the sheet S. The movable sheet guide 72 is provided with an opening through which the auxiliary roller 71 can be lifted and lowered.
The sheets S stacked on the intermediate tray 29 are fed from an uppermost one by the auxiliary roller 71 rotated in the anti-clockwise direction in FIG. 11. In this case, the auxiliary roller 71 is lowered from the waiting position shown by the solid line to a position shown by the broken line to convey the sheet S. The movable shutter 44 is laid toward an inclined condition to guide the supplied sheet S to a nip of a pair of separation rollers 47. A side guide 80 shiftable in accordance with the size of the sheet S is provided on the intermediate tray 29 to prevent the skew-feed of the sheet.
The sheets S fed by the auxiliary roller 71 are separated by the pair of separation rollers 47 so that only the uppermost sheet is conveyed to the pair of regist rollers 13 (which are now stopped) by the pair of convey rollers 9 to 12. Further, a detection means 81 for detecting presence/absence of the sheet S is provided on the intermediate tray 29. By detecting the absence of the sheet S after the re-supply of the sheet is finished, the double-feed of the sheets is checked.
Explaining with reference to FIG. 11, in the both-face mode, the sheet S sent into the vertical path 23 is sent to the convey-in path 27 by the pairs of convey rollers 24 to 26 and then is sent to the discharge path 60 by the pair of convey-in rollers 28. In this both-face mode, all of the flappers 66 to 70 in the discharge path 60 were rotated downwardly. The sheet S sent into the discharge path 60 is conveyed to a terminal U-turn portion 73 by the pair of discharge rollers 61 to 65 and then is discharged onto the intermediate tray 29 by a discharge roller 74 and a discharge sub-roller 75, with an imaged surface of the sheet facing upwardly. In this case the tip end of the sheet is guided by the movable sheet guide 72 set to an inclined condition as shown by the solid line to suppress the curl.
The sheet discharged on the intermediate tray 29 is conveyed by the auxiliary roller 71 rotated in the anti-clockwise direction in FIG. 11 until the tip end of the sheet abuts against the rotatable shutter 44 which is not cocked. In this case, the tip end of the sheet is guided by the movable sheet guide 72 set to the inclined condition as shown by the solid line to suppress the curl. The auxiliary roller 71 is positioned at the upper waiting position when the sheet is discharged to permit the discharging of the sheet, and, immediately after the sheet was discharged, the roller 71 is lowered to convey the sheet S. Similar to the multi-mode, the sheets S stacked on the intermediate tray 29 are successively re-supplied toward the pair of regist rollers 13 from an uppermost one. Incidentally, while the sheet is being dropped toward the intermediate tray 29, the auxiliary roller 71 is lowered from the waiting position, thereby forcibly urging the sheet against the intermediate tray 29.
However, in the above-mentioned conventional example, both in the both-face mode and multi-mode, when the sheet S discharged on the intermediate tray is urged against the shutter 44 by the auxiliary roller 71, the sheet is stopped by stopping the rotation of the auxiliary roller 71 or separating the auxiliary roller from the sheet S. In the conventional technique, a timing for stopping the sheet is selected as a fixed value determined by the sheet size in the multi-mode or as a uniform fixed value in the both-face mode. In this arrangement, the dropped position of the sheet is varied due to unevenness (between apparatuses) in response to a shifting speed and a shift start timing of a means for abutting the auxiliary roller 71 against the sheet S, and the dropped position is also varied due to the change in the response and the shifting speed caused by the change in environment and endurance. Further, since the curl in the sheet is greatly changed in accordance with the kind of the sheet and the environment and since the resiliency of the sheet is changed in accordance with the kind of the sheet, the flying speed of the sheet is changed by such factors, thereby changing the dropped position of the sheet.
Further, the position of the auxiliary roller 71 stopped on the intermediate tray with the interposition of the sheets is changed in accordance with the number of sheets stacked on the intermediate tray. Further, the distance through which the sheet is conveyed by the auxiliary roller 71 is changed due to unevenness in a diameter and a coefficient of friction of the auxiliary roller 71 and due to change in such factors during time lapse. Thus, the following problems arise.
Since the auxiliary roller 71 has a conveying force sufficient to surely convey the sheet S to the pair of convey rollers 47, if the sheet S is further conveyed after the sheet S reaches the movable shutter 44 due to the unevenness in the dropped position of the sheet S and/or the unevenness in a conveying distance of the auxiliary roller 71, the sheet S is buckled to generate a folding line in the sheet. Further, when the buckling of the sheet S is released by separating the auxiliary roller 71 from the sheet S, the sheet is flying away from the movable shutter 44, thereby causing mis-alignment of the sheets on the intermediate tray.
In addition, even if the buckling is not generated, in the both-face mode, since the auxiliary roller 71 is slid on the imaged surface of the sheet S, the image is distorted and/or the auxiliary roller 71 is contaminated, with the result that further sheets are contaminated by the auxiliary roller. Further, if the sheet S does not reach the movable sheet 44 due to the unevenness in the dropped position of the sheet S and/or the unevenness in the conveying distance of the auxiliary roller 71, the mis-alignment of the sheets will occur.